Spark plug

Abstract

A spark plug includes a center electrode; a metal shell that retains the center electrode at an outer periphery of the center electrode in an insulating manner; a ground electrode disposed such that a spark gap is formed between the center electrode and an end portion of the ground electrode; and a plug cap connected to the metal shell, the plug cap covering the center electrode and the end portion of the ground electrode from front and having a through hole in a region in front of the ground electrode. An inner surface of the plug cap has at least one ridge in a first region that is in front of an inner open end of the through hole.

What is claimed is: 
     
       1. A spark plug comprising:
 a center electrode; 
 a metal shell that retains the center electrode at an outer periphery of the center electrode in an insulating manner; 
 a ground electrode disposed such that a spark gap is formed between the center electrode and an end portion of the ground electrode; and 
 a plug cap connected to the metal shell, the plug cap covering the center electrode and the end portion of the ground electrode from front and having a through hole in a region in front of the ground electrode, 
 wherein an inner surface of the plug cap has at least one ridge in a first region that is in front of an inner open end of the through hole. 
 
     
     
       2. The spark plug according to  claim 1 , wherein a size of the ridge is such that a length of the ridge in a circumferential direction of the inner surface is greater than a length of the ridge in an axial line direction of the inner surface. 
     
     
       3. The spark plug according to  claim 1 , wherein the ridge is continuous over an entire circumference of the inner surface. 
     
     
       4. The spark plug according to  claim 1 , wherein the inner surface of the plug cap additionally has the ridge in a second region that is in front of the ground electrode and behind the inner open end.